1. Field of the Invention
Demands for ever faster transfer of signals in digital computers have led to the advent of three-dimensionally structured microelectronic semiconductor devices for use in such computers. One such three-dimensionally structured device is disclosed in U.S. Pat. No. 4,275,410, by Grinberg et al. In that device, signals are transferred through stacked semiconductor wafers each of which is interconnected with adjacent wafers by external leads extending from both surfaces of the wafer. Transfer of signals through each wafer is provided by internal connections which are positioned between the wafer surfaces. The process for fabricating such internal connections is the subject of the present invention.
2. Description of the Prior Art
One process for fabricating such internal connections is a technique generally called temperature gradient zone melting; an example of which is disclosed in U.S. Pat. No. 3,897,277, by Blumenfeld. In this process, a liquid zone migrates in a solid along a thermal gradient. In particular, a liquid droplet of metallic material placed on one surface of a solid wafer migrates through the solid and emerges on the wafer's second surface. The wafer is heated to an elevated temperature for establishing a thermal gradient through the wafer, the hotter temperature being at the second surface. The thermal gradient causes the temperature at the forward interface of the droplet to be higher than the temperature at its rear interface. Since the solubility of a solid in a liquid increases with temperature, the concentration of the atoms from the dissolved wafer solid is greater at the hotter forward interface of the liquid than at the cooler rear interface. The inequality in concentration produces a concentration gradient of atoms from the dissolved wafer solid across the liquid droplet. The concentration gradient, in turn, creates a flux of atoms from the dissolved wafer solid along this gradient, that is, a flow of atoms from the hotter front interface to the cooler rear interface. To feed this diffusion flux, additional atoms of the wafer solid are dissolved into the liquid at the forward interface and moved to the rear interface. Consequently, the liquid droplet migrates along the thermal gradient toward the hotter second surface, dissolving atoms of the wafer at its forward interface, passing the atoms toward the rear, and redepositing these atoms at its rear interface. At the cooler rear interface, the redeposited atoms recrystallize with traces of the droplet metallic material in them. Thus, the path left by the migrating liquid droplet is higher in conductivity than rest of the wafer. The conductive path, extending from one wafer surface to another, is the internal connection necessary for the operation of a three-dimensional microelectronic device. However, prior art processes employing the temperature gradient zone melting technique are deficient in several aspects.
One deficiency in the prior art is the inability of the prior art processes to produce a high thermal gradient through the wafer. Since the thermal gradient in a wafer is generally proportional to the quantity of heat that flows through the wafer and correspondingly the migration rate of the droplets is generally proportional to the thermal gradient, the quantity of heat flowing through the wafer should be as high as possible. However, both the duration that the wafer is exposed to the heat and the quantity of heat itself invariably cause defects in the wafer. Thus, creating a high thermal gradient through a wafer without using an even greater quantity of heat would result in the twin benefits of faster migration rate and less exposure to heat, producing more defect-free wafers.
One prior art process of enhancing the thermal gradient through a wafer is disclosed in U.S. Pat. No. 4,033,786, by Anthony et al., in which anti-reflection coatings are applied to wafer surfaces for trapping heat in the wafer. However, this and other prior art processes are still not efficient because each of them provides a gas-filled gap between the wafer and the heating source. The gas usually acts as an insulator in preventing the most efficacious transfer of heat from the heat source to the wafer, thereby requiring either more heat to flow through the wafer or longer exposure to heat in order to create the necessary thermal gradient. Examples of such processes are disclosed in U.S. Pat. No. 3,895,967, by Anthony et al. and U.S. Pat. No. 3,910,801, by Cline et al.
Another deficiency in the prior art is the inability of the prior art processes to provide a uniform temperature gradient through the wafer, thereby causing nonuniform droplet migration. Uniformity refers to the capability of providing both parallel heat flow lines for parallel migration of droplets and linear isotherms parallel to the wafer surfaces which indicate the establishment of the same temperature at a given depth in the wafer. Uniform droplet migration is essential in order to maximize the number of completed migrations within a certain process time limit, a completed migration being a generally vertical internal connection with exposed ends on both surfaces of the wafer. Without such uniformity, the yield of usable conductive internal connections in each wafer decreases.
The parallel heat flow line type of nonuniformity is usually caused by support pins or holders that are necessary for supporting the wafer in the heating apparatus. The pins or holders, acting like heat sinks because they are more heat conductive than the surrounding gas, create deviations in the parallel heat flow lines through the wafer. Examples of the use of such pins and holders are described in the processes disclosed in U.S. Pat. No. 3,895,967, by Anthony et al. and U.S. Pat. No. 4,001,047, by Boah. To alleviate this problem, support ribs and guard rings around the periphery of the wafer are used to prevent lateral heat flow. Such devices are used in processes disclosed in U.S. Pat. No. 3,895,967, by Anthony et al. and U.S. Pat. No. 4,035,199, by Anthony et al. respectively.
The linear isotherm type of nonuniformity occurs in infrared heating processes such as the process disclosed in T. R. Anthony and H. E. Cline, "Stresses Generated by the Thermomigation of Liquid Inclusions in Silicon," Journal of Applied Physics, Vol. 49, No. 12, December, 1978. Because silicon is semi-transparent to infrared radiation, infrared radiation is absorbed at varying depths in the wafer. The absorbed radiation, thus, provides a variety of temperatures at a given depth of the wafer, creating nonlinear isotherms through the wafer. In turn, the nonlinear isotherms created by an infrared process cause non-uniform migration of the droplets. In contrast, non-infrared processes provide the same temperature at a given depth of the wafer, thereby creating linear isotherms which are parallel to the wafer surfaces. Aluminum droplets therefore, migrate through a uniform distance of the wafer from a lower-temperatured isotherm to a higher-temperatured isotherm during the same time period.
The prior art processes, thus, are deficient in their inability to produce a high thermal gradient through the wafer and to provide a uniform temperature gradient through the wafer.